JPS6129588B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a television receiver using an intercarrier audio system, and more particularly to a device for obtaining an angle-modulated intercarrier audio signal in a color television receiver.

In a color television receiver that uses an intercarrier audio system, the image and audio intermediate frequency carrier waves are amplified in a common IF channel, and the amplified carrier waves are fed to a mixing circuit where they are synthesized. A modulated intercarrier audio IF signal is generated. This intercarrier audio IF signal is obtained by intermodulating an audio carrier wave and an image carrier wave, is amplified by an audio IF amplifier, is amplitude limited, and is supplied to a frequency modulation detector for audio information recovery. These audio and video IF carriers are also fed to the video detector by a tuned trap circuit. This trap circuit removes the audio carrier from the IF signal. The video detector recovers the video information and sends it to the color television receiver's luminance processing, chrominance processing, and deflection processing systems for further processing.

Although the basic intercarrier audio scheme described above produces the desired 4.5 MHz intercarrier audio signal under most operating conditions, a problem arises when it inadvertently detects a video signal on the image carrier. To generate an intercarrier audio signal, the IF audio and image carriers are combined or mixed by a rectifier (e.g., a diode or base-emitter junction of a transistor), which rectifies the intercarrier audio signal. In addition to generation, it also operates as a video detector for image carrier waves and detects amplitude modulated video signals. This detected video signal is sent to the output terminal of this rectifier at 4.5MHz.
It occurs together with the intercarrier audio signal, and changes the DC level of the intercarrier audio signal in accordance with the amplitude of the detected video signal.

The intercarrier audio channel typically includes a high Q circuit tuned to 4.5 MHz prior to the FM audio detector to select the intercarrier audio signal to contain as little of the accompanying video signal as possible. By feeding the output of this audio mixer directly to this 4.5MHz tuned circuit before amplifying it, the detected video signal can be removed from the intercarrier audio signal before it has a negative effect on the operation of this audio device. .

However, when this mixer is included in an integrated circuit in a television receiver, the intercarrier audio signal is amplified on an integrated circuit board that is easy to amplify, and the high-level intercarrier audio signal is transferred to the circuit board. It is generally desirable to make the information available outside the organization. RCA Co., Ltd. (RCA
In a typical integrated circuit, such as the Model CA3139E Television Automatic Fine Tuning Intercarrier Mixing Amplifier from Corp., the intercarrier audio signal is mixed and immediately amplified to provide the signal from the integrated circuit to the audio detector. becomes a high-level audio IF signal. Therefore, this audio IF signal may not need to be amplified before amplitude limiting and detection.

However, since the detected video signal at the output terminal of this mixer is not removed before being amplified by the integrated circuit amplifier, it is amplified by the integrated circuit amplifier of this mixer, but if the video signal has a considerable amplitude If so, the mixer's integrated circuit amplifier can become overloaded and distort or even cancel the intercarrier audio signal. Peaks in the video signal can cause the amplifier in the mixer's integrated circuit to become saturated or cut off, resulting in intercarrier audio signal distortion components that repeat at 60Hz (vertical frequency), resulting in amplitude limiting and FM detection. Sometimes it's so bad that you can't do it at all. Such distortion produces an audible 60 Hz buzz in the television receiver's speakers. Also, to prevent amplitude modulation of the intercarrier audio signal by the amplitude-varying image carrier, the amplitude of the image carrier at the input of the mixer is set to be substantially larger than the amplitude of the audio carrier (i.e., 25 to
30 db), so there is always the possibility of an overload condition occurring due to the large amplitude of this video signal.

An intercarrier audio amplifier according to the principles of this invention includes an intercarrier audio mixer and an audio amplifier.
One transistor is also used as an IF amplifier. The transistor has an input circuit including a base-emitter junction and an output circuit including a collector electrode, the image carrier being supplied to the transistor by the input circuit and mixed via its base-emitter junction. An electrode common to the input and output circuits of the transistor is provided with means whose impedance changes inversely to changes in the signal frequency applied to the input circuit, and the variable impedance means provides negative signal feedback of the transistor. The intercarrier audio signal is amplified to a greater extent than the low frequency components of the intercarrier audio signal generated at its collector.

Hereinafter, this invention will be explained based on the drawings.

In FIG. 1, a television signal from a broadcast station is received by an antenna 10 and supplied to a tuner 20. In FIG. Tuner 20 selects one frequency range from a number of broadcast frequencies in the high frequency band.
The received broadcast signal is heterodyne detected in this tuner by a signal from a local heterodyne oscillator to generate sum and difference frequencies between the original radio frequency and the local oscillation frequency. These signals are fed to selection circuitry 30 where all but the difference frequency, called the intermediate frequency (IF), is wave-cancelled. The resulting IF signal includes the audio carrier, the image carrier, and the chrominance subcarrier as well as some of their sidebands that have passed through selection circuitry 30. The amplitude of this audio carrier is approximately 10 db lower than the amplitude of the unmodulated image carrier at the leading edge of the synchronization signal.

The IF signal at the output terminal of the selection circuitry 30 is the prefix IF
The signal is supplied to an amplifier 40. This pre-IF amplifier 4
0 includes one or more amplification stages to raise the level of the IF signal by approximately 6 mV for the audio carrier and approximately 20 mV for the image carrier at the leading edge of the synchronization signal. The amplified IF signal is sent to an intercarrier audio converter 100 and a 41.25MHz trap 5.
0. The 41.25 MHz trap 50 includes a wave circuit tuned to the frequency of the audio carrier to sharply attenuate the audio carrier relative to the image carrier and chrominance subcarrier. To prevent mixing of the 3.58 MHz chrominance subcarrier and the 4.5 MHz audio carrier in the video detector 70, it is desirable to attenuate the audio carrier before providing the IF signal to the final IF amplifier 60. When the chrominance subcarrier and the audio subcarrier are mixed in this video detector 70, 4.5MHz-
A beat signal of 3.58MHz = 920KHz is generated, which appears visually on the screen of the picture tube.

The IF signal at the output terminal of the 41.25MHz trap is the final
The signal is supplied to an IF amplifier 60, where it is further amplified and supplied to a video detector 70. This video detector 70 detects the amplitude modulation of image transmission and supplies a video signal to a video amplifier 80. This video amplifier 8
0 increases the level of the video signal for further processing by the television receiver's luminance and chrominance processing equipment and deflection equipment. The amplified IF image carrier generated by this final IF amplifier 60 varies in level from 1V at the tip of the sync signal to 100mV at the highest modulation level (white level).

The IF signal produced by pre-IF amplifier 40 is applied via conductor 42 to the base electrode of emitter follower transistor 102. The collector electrode of the transistor 102 is connected to a voltage (+B) source, and the emitter electrode is connected to the base of a transistor 104 and via a resistor 103 to a reference voltage source (ground point). At the base electrode of transistor 104, the audio carrier exhibits a substantially constant amplitude of approximately 6 mV rms, whereas the amplitude of the image carrier exhibits a substantially constant amplitude at the leading edge of the synchronization signal.
It varies from 20mV to an effective value of 2mV at the white level.

The emitter electrode of transistor 104 is grounded, and the collector electrode is connected to the emitter electrode of mixed transistor 106. transistor 10
The emitter electrode 6 is grounded via a capacitor 108. Transistor 104 acts as a current source for transistor 106 and supplies mixing transistor 106 to intertone (mix) the audio carrier with the image carrier. Transistor 104 and capacitor 108 together provide a power supply impedance to transistor 106.

The amplified image carrier generated by final IF amplifier 60 is routed through conductor 62 to transistor 106.
is supplied to the base electrode of transistor 106
The level of the image carrier at the base electrode varies over a range of 20 db from an effective value of 1 V at the tip of the synchronization signal to an effective value of 100 mV at the white level.

The collector electrode of mixed transistor 106 is connected to the +B power supply via resistor 112 and capacitor 114. The signal at the collector electrode of transistor 106 is the emitter follower transistor 1.
16 bases are supplied. transistor 116
The collector electrode of is connected to the +B source, and the emitter electrode is grounded through a resistor 118. It is also connected to the base of transistor 126 via the emitter electrode of transistor 116 or series resistors 122, 124, and a capacitor 130 is connected between the base of transistor 126 and the +B power supply.

The collector electrode of transistor 126 is connected to the +B power supply, and the emitter electrode is grounded through resistor 128 and connected through capacitor 132 to the connection point of resistors 122 and 124.
generated at the emitter electrode of transistor 126
4.5MHz intercarrier audio signal is connected to resistor 14
2 to the output terminal 140.

The audio and image carrier signals applied to the base electrode of transistor 104 have sufficient amplitude to maintain linear operation of transistor 104, so that transistor 104 is able to provide an audio carrier to the emitter electrode of transistor 106 when that transistor 106 current source. In the absence of a carrier signal, transistor 104 shifts the static operating point of transistor 106 to transistor 1, as shown in FIG.
Set at a low level in the forward bias region as indicated by point X on the base-emitter transmission characteristic curve of 0.06.

Curve 150 in FIG. 2 represents a typical base-emitter transfer characteristic of transistor 106. When a relatively high level image carrier is applied to the base electrode of transistor 106 via conductor 62, the transistor becomes non-linear, with its base-emitter junction following characteristic curve 150 from cutoff point 154 to the forward bias region at point 156. Makes a straight line movement. This non-linear operation causes intermodulation between the audio carrier wave and the image carrier wave at the base-emitter junction of this transistor, resulting in an amplified 4.5MHz intercarrier beat at its collector electrode.

The audio and image carriers also appear in amplified form at the collector electrode of transistor 106 and require removal. These carrier frequencies are connected to the collector of transistor 106 and output terminal 140.
is removed by two low-pass filters provided between the two. First RC low pass waver 110
consists of a resistor 112 and a capacitor 114 connected to the collector electrode of transistor 106.
This RC low-pass filter 110 exhibits a response characteristic in which signal attenuation gradually increases as the frequency increases, and attenuates the audio carrier (41.25 MHz) and image carrier (45.75 MHz) signals.

The signal that has passed through the low-pass wave generator 110 is passed through the transistor 116 to the second low-pass wave generator 12.
0. This low-pass wave device 120 is an active wave device consisting of circuit elements 122 to 132, including a transistor 126 and resistors 122 and 124.
A Chebychev response characteristic is provided by inserting a capacitor 132 between the connection point of . This response characteristic is
It is characterized by being essentially flat up to frequencies just above the 4.5MHz intercarrier audio signal frequency, and exhibiting a sharp roll-off (attenuation) characteristic above the 4.5MHz intercarrier audio signal frequency. The low pass filters 110, 120 thus serve to significantly attenuate signals at the audio and video carrier frequencies.

The nonlinear operation of transistor 106 provides mixing of the audio and image carriers, but also detects the low frequency video signal component of the image carrier at the base-emitter junction of transistor 106. This video signal component has a horizontal line frequency (approximately 15.734KHz) and a vertical field frequency (approximately 60KHz).
Hz) and television synchronization signal components. Horizontal line frequency modulation has a frequency that is too high and does not cause problems with the audio system, but vertical frequency modulation requires transistor 1
06 will produce an audible buzz if allowed to overload. However, the low frequency video signal component at the base-emitter junction of transistor 106 is
Because of the negative feedback of the emitter of 6, it does not occur in an amplified form at its collector. Due to this emitter negative feedback, the transistor 106 attenuates or only slightly amplifies the low frequency components of the video signal, but the relatively high frequency 4.5MHz intercarrier audio signal and the video/audio carrier wave are substantially Can be amplified.

The attenuation of this low frequency signal is caused by the mixed transistor 10
This can be explained by examining the amplification characteristics of No. 6. The gain of transistor 106 is calculated by dividing the load impedance of its collector by the source impedance of its emitter. Transistor 106 has a relatively small load (collector) impedance and a relatively large source (emitter) impedance at low frequencies, and exhibits low gain for low frequency signals. For example, for a DC signal, this load impedance is the value of resistor 112 (10K in the embodiment of FIG.
Ω), and the source impedance is equal to the collector output impedance (100KΩ) of the transistor 104, which acts as a high impedance constant current source at low frequencies. Therefore transistor 106
The gain of is 0.1 for DC signals. The transistor 104 cannot be said to be a constant current source for high frequency signals, and its impedance decreases. Moreover, the reactance of capacitor 108 decreases at higher frequencies, and the reactance of transistor 104 and capacitor 1 decreases at higher frequencies.
The overall effect with 08 is a reduction in the emitter impedance of transistor 106 at high frequencies. The load impedance of transistor 106 also decreases at high frequencies due to the effect of capacitor 114, but the rate of decrease is substantially less than the rate of decrease of the source impedance. Therefore, the transistor 106 operates as a negative feedback amplifier in which the amount of negative emitter feedback is large at low frequencies and small at high frequencies, so that the transistor 106 increases the amount of high frequency signals more than the low frequency signals. Amplify. Therefore transistor 1
The signal at the collector electrode of 06 is substantially free of low frequency audible amplitude signal components. In this way, mixing transistor 106 can handle intercarrier audio signals at 4.5 MHz without overloading or signal distortion.
It is amplified before being waved by a bandpass wave generator (not shown).

The audio converter 100 of the present invention benefits from using the high level image carrier provided by the final IF amplifier 60 as a mixing signal. In other words, this high level signal
06, and even at the minimum amplitude (100 mV), the amplitude of the image carrier in conductor 62 is more than the amplitude of the audio carrier in conductor 42 (6 mV). Because of its large size, undesirable amplitude modulation of the intermodulated intercarrier audio signal can be minimized.

The audio transducer 100 of the present invention can be modified so that both the audio carrier and the image carrier are supplied to the base electrode of transistor 106, as shown in FIG. In this configuration, transistor 1
The collector of transistor 04 is connected to the base of transistor 106 instead of its emitter. In place of the transistor 104 serving as the source impedance of the transistor 106, a 100KΩ resistor 160 is inserted between the emitter of the transistor 106 and the ground, thereby maintaining the DC source impedance of the emitter electrode at 100KΩ. The image carrier wave is provided from conductor 62 through a decoupling resistor 162 in series therewith to the base of transistor 106. This decoupling resistor 162 is connected to the transistor 10 which is responsible for producing a 920KHz visible beat signal by mixing the 4.5MHz audio carrier and the 3.5MHz color subcarrier in the video channel.
4 collector to the final IF amplifier 60. This backflow of the audio carrier to the final IF amplifier is further reduced by the buffer stage drive conductor 62 from the final IF amplifier 60.

In the audio converter 200 of FIG. 3, the collector load resistor 112 of the transistor 106 is 10KΩ.
10 of the load resistor 112 is used as the emitter negative feedback resistor 160.
A double 100KΩ resistor is used. Also,
As the capacitor 108, a capacitor with a small capacity of 5 pf is used, which exhibits high impedance in a low frequency range of about 920 KHz. Therefore, the parallel circuit of the emitter resistor 160 and the capacitor 108 essentially operates as a constant current source in the low frequency range of about 920 KHz, and therefore the audio converter 200 including the transistor 106 operates as a constant current source in the low frequency range of about 920 KHz. It has virtually no amplification effect.

However, at the intercarrier audio signal frequency of 4.5 MHz, the impedance of capacitor 108 is as low as approximately 7KΩ, and audio converter 200 can amplify the intercarrier audio signal with the required gain.

For the circuits shown in Figures 1 and 3, 41.25M
Final IF amplifier 60 to transistor 10 for ineffective trapping of the audio carrier by Hz.
Some residual portion of the audio carrier may be applied to the base of 4. This residual audio carrier may be in phase relationship with the audio carrier provided by transistor 104 such that the audio carrier is attenuated or canceled by transistor 106. This undesirable effect can be prevented by maintaining the level of the audio carrier signal at the bases of transistors 102, 104 sufficiently high (eg, 5-6 mV). Due to the high level audio carrier provided by transistor 104, any residual audio carrier provided from final IF amplifier 60 will only slightly attenuate the intermodulated audio carrier at transistor 106. If the 41.25MHz trap is on the audio carrier and ±50K outside of that carrier
Broadband tuning to eliminate sidebands down to Hz eliminates this slight attenuation of the audio carrier.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a first embodiment of the intercarrier audio system according to the present invention, and FIG.
FIG. 3 is a block diagram of a second embodiment of the intercarrier audio system of the present invention. 106... Amplifying device, 104, 108... Amplification degree changing means.

Claims (1)

[Claims] 1 An amplifier device having a base electrode, an emitter electrode, a collector electrode, and a rectifying junction between the base electrode and the emitter electrode, and an audio carrier signal and a video signal that include the base electrode and the emitter electrode and are frequency modulated with audio information. an input signal path for supplying an amplitude modulated image carrier signal to the rectifying junction; and an output signal path comprising the collector electrode and one of the base electrode and the emitter electrode for producing an amplified signal comprising an intercarrier audio signal. is coupled to the input signal path and the output signal path, and has an impedance that changes in the opposite direction to a change in frequency, and has an impedance that changes in the opposite direction to a change in frequency, and adjusts the amplification degree of the amplification device between the video signal and the intercarrier audio signal. a substantially constant current source at low frequencies, the modulated audio and image carrier signals are synthesized to produce an intercarrier audio signal. An intercarrier audio amplification device for television receivers that generates